Browsing Images in World Coordinate Space with SAOimage

The availability over the Internet of many large-scale astronomical
surveys, such as the IRAS Infrared Sky Survey (Van Buren et al 1995),
and the HST Digitized Sky Survey (Morrison 1995), as well as the Hubble
Space Telescope archives (Travisano 1995) was discussed at the 1994
Conference on Astronomical Data Analysis Software and Systems.
In addition to such World Wide Web interfaces to specific data sets,
SkyView (McGlynn et al 1995) provides a versatile interface to data from
many surveys.

Information linking spatial coordinates (world coordinate system or WCS)
to image pixel coordinates is inserted into the headers of the FITS
images derived from these surveys. Descriptions of standard projective
systems are now being standardized (Greisen and Calabretta 1995a, 1995b).
The Digitized Sky Survey, scanned from photographic plates rather than
created from digital detectors, provides plate solutions with each extracted
image. All that was needed was an easy-to-use viewing program which could
display WCS information.

SAOimage was developed in the late 1980's at the Smithsonian Astrophysical
Observatory to display astronomical images (Van Hilst 1990). Its ease of use,
simplicity, and portability have made it popular throughout the astronomical
community.

When the wealth of available WCS-containing FITS-format information became
apparent, steps were taken to add world coordinate access to
SAOimage. Both cursor tracking in sky coordinates (see figure 1) and the
ability to export cursor positions as sky coordinates were added to the
program. Additional capablities have been added as users requested them
and new data formats became available.

Figure 1:
SAOimage displays world coordinates at the cursor position,
in this case FK4/B1950 right ascension and declination, above the image
pixel coordinates

SAOimage was first modified in version 1.09 to extract WCS
parameters from FITS image headers. A WCS data structure was
developed, and the worldpos C subroutines by Bill Cotton and Don Wells,
which implement the eight projections from classic AIPS (see Table 1),
were modified to work
with it. Subroutines to initialize the data structure and to convert WCS
cursor positions into character strings completed a self-contained library.
Digitized Sky Survey plate solution algorithms were added in version 1.12.
As an example of its portability, this
library is also being used in a new image display program, SAOtng
(Mandel 1995).

Table 1:
World coordinate systems supported by SAOimage

CTYPE value

Projection

Comment

-AIT

Hammer-Aitoff

equal-area all-sky

-ARC

Zenithal equidistant

closest to a Schmidt plate

-CAR

Cartesian

linear, default

-GLS

Samson-Flamsteed

global sinusoidal projection

-MER

Mercator

-NCP

North Celestial Pole

polar

-SIN

sinusoidal

orthographic

-STG

Stereographic

zenithal orthomorphic projection

-TAN

tangent plane

gnomonic

The option of displaying or exporting coordinates in systems other
than that used in which the image header was added in version 1.14.
Subroutines derived from Patrick Wallace's SLALIB positional astronomy
library (Wallace 1994) were used to translate between FK4, FK5, and galactic
coordinate systems. Version 1.17 extended WCS access to IRAF OIF images,
by internally translating IRAF headers to FITS headers and
extracting parameters using the code already used for FITS files.

The "c" command was added in SAOimage version 1.09 to print
the current cursor position in world and image pixel coordinates, along
with the value of the closest pixel. Right ascension and declination
are printed as

hh:mm:ss.sss dd:mm:ss.ss;

galactic latitude and longitude are printed as

dd.ddddd dd.ddddd.

If WCS information is not available in the header, only pixel
coordinates and value are returned.

In version 1.14, commands were added to SAOimage to print the world
coordinates of the current cursor position in any of three coordinate
systems. Coordinates will be converted if this is not the same system
as that in the image header.
b prints the current cursor coordinates in the B1950 (FK4) system as
hh:mm:ss.sss dd:mm:ss.ss B1950 .
j prints the cursor coordinates in the J2000 (FK5) system as
hh:mm:ss.sss dd:mm:ss.ss J2000 .
g prints the cursor position in galactic coordinates as
dd.ddddd dd.ddddd galactic .
If SAOimage is being used to look at a local image, output will be to
the window from which SAOimage was invoked. If SAOimage is being used
with Mosaic, output is sent to the console window. Netscape sends
output from helper applications to a new window, but not until several
lines have been sent.

Additional command line flags set the format in which the world coordinates
of the current cursor position will be displayed.
The same position string that is displayed above the pixel coordinate in the
SAOimage window is sent to standard output when the "c" command is used and
to an external program when the "w" cursor command is issued.

Adding the switch -fk4 or -b1950 to the command line
causes the cursor to track in FK4/B1950 right ascension and declination.
The coordinates
in hours, minutes, and seconds of right ascension and degrees, minutes, and
seconds of declination will put into a string of the format
hh:mm:ss.sss dd:mm:ss.ss B1950 .

The switch -fk5 or -j2000 makes the cursor track in
FK5/J2000 right ascension and declination, displaying
hh:mm:ss.sss dd:mm:ss.ss J2000 .

The command line switch -galactic (which may be shortened to
-gal) causes the cursor to track in galactic longitude and latitude.
The coordinates in decimal degrees will be displayed to five decimal
places in the format dd.ddddd dd.ddddd galactic . Leading
zeroes are dropped.

To set the external command to be executed, add -wcscom followed
by a space-free string containing a Unix command string to the command line.
The world coordinate string will be substituted for %s in the
Unix command string, as in C formatted output.
Underscores should be used instead of spaces.
The "w" cursor command executes the command specified on the command line
using -wcscom, passing the string corresponding to the current cursor
position.

To search the Hubble Space Telescope Guide Star Catalog in a
30-arcsecond-square region centered on the current cursor position using
the RGSC program, add
-wcscom rgsc_%s_-box_0:00:15 to the saoimage command.
When the cursor is centered on the star in the upper right corner of
figure 1, the "w" command prints the following:

SAOimage can be used as a "helper" application to a World Wide Web
browser such as Netscape or Mosaic.
To make sure SAOimage is used whenever a FITS image is encountered,
this line should be added to your .mime.types file:

image/x-fits fit fits fts FIT FITS

and this line to your .mailcap file

image/x-fits; saoimage -fits %s

To serve FITS files from a http server so that SAOimage will
be invoked to browse them, add this line to the httpd srm.conf file:

AddType image/x-fits .fit .fits .fts .FIT .FITS

and make sure that any FITS files on your server have one of those extensions.